I know I am a day late. Success of the D-day landings was crucial to the defeat of Hitler. It was a humungus enterprise, thousands of landing craft, all built just in time, a million soldiers, tanks, floating harbors, fuel pipelines laid across the channel, and a zillion other things cranked out by British and American industry.
It was extremely dangerous. It might have failed. It was so chancy that Eisenhower, the supreme commander, and the man with the best information, penned a press release announcing defeat of the invasion. Fortunately he never had to use it. With a little more luck, and a better German command structure, Rommel might have been able to throw German armor into the battle on the day of the landings instead of a day later.
Defeat would have been a disaster. It took all of 1942 and 1943, and half of 1944 to build up for D-day. After a defeat on the landing beaches, it would have taken at least another year to build up to a second try. Hitler would have been able to move all the troops guarding France against invasion to the Russian front and that surely would have slowed the Red army down, perhaps even defeated it. It would have given Hitler time to bring secret weapons, V1, V2, jet fighters, guppy submarines, even nuclear weapons into action.